This had been reported in an evolution-list mailing list:
Users cannot modify existing meeting with 3.28.3 version. It had been introduced in a fix for upstream bug:
which turned out to have a bog and also it is incomplete. An upstream fix exists and can be easily backported.
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.